1. Field of the Invention
The present invention relates to an apparatus for transmitting a radio signal and especially relates to a radio base station apparatus for implementing signal transmission having directivity using an adaptive array antenna.
2. Description of the Related Art
At present, the operation of standardizing the third generation mobile communications system has been pursed in the 3rd Generation Partnership Project (3GPP).
FIG. 1 is a block diagram of a network related to the 3GPP system. The 3GPP system comprises an upper level network (core network (CN)) 100, a radio access network (UTRAN: UMTS (Universal Mobile Telecommunications System) terrestrial radio access network) 110 and a mobile station (user equipment (UE)) 120. Here, each radio access network 110 is provided with a radio network controller (RNC) 111 and a plurality of radio base stations (NodeBs) 112.
The interface between the radio base station (NodeB) 112 and the user equipment (UE) 120 is named as [Uu], and the physical bearer is radio transmission. Furthermore, the interfaces between the radio base station (NodeB) 112 and the radio network controller (RNC) 111, between the radio network controller (RNC) 111 and the core network (CN) 100 and between the radio network controllers (RNCs) 111 are named as [Iub], [Iu] and [Iur], respectively. The physical bearers are fixed lines. In the above-mentioned system, it is defined that a direction from the core network (CN) 100 to the user equipment (UE) 120 is a downlink (DL), while the direction from the user equipment (UE) 120 to the core network (CN) 100 is an uplink (UL).
In the above-mentioned system, the user equipment (UE) 120 is accommodated in the nearest radio base station (NodeB) 112 and receives/transmits data from/to the other terminal via the radio base station (NodeB) 112.
In a radio base station of the 3GPP system, an adaptive array antenna is often provided. The adaptive array antenna is provided with a plurality of antenna elements. By appropriately controlling the weight of transmission electric power of each antenna element, radio communications having directivity as shown in FIG. 2 can be implemented. That is, when a signal is received from a user equipment, the weight of each antenna is appropriately controlled so as to optimize a reception property. Consequently, a directivity pattern can be offered by directing a main lobe in the coming direction of a desired wave signal and by directing a beam null in the coming direction of an interference wave. In this way, the effect of an interference wave is decreased and high-quality data can be received so that the capacity of an uplink increases.
In the transmission of a signal to a user equipment, on the other hand, a directivity pattern such that a main lobe is directed toward the position of a target user equipment by appropriately controlling the weight of each antenna element for each user equipment, is offered. Thus, the interference given to the other user equipment can be decreased so that the capacity of a downlink increases.
FIG. 3 is a block diagram of an adaptive array antenna system. In the adaptive array antenna system, an adaptive processor 130 calculates a weight for each antenna element in order to optimize the quality of a composite signal obtained by composing signals received via the plurality of antenna elements 1a through 1n. At this time, the weight for an antenna element corresponding to the coming direction of a desired wave is weighed, while the weight for an antenna element corresponding to the coming direction of an interference wave is reduced. In the transmission of a signal to a user equipment, a transmission beam having directivity is formed using the weights calculated based on the receipt signal from the user equipment.
In this way, in the adaptive array system, the weight for each antenna element is calculated based on the receipt signal from a user equipment and a signal is transmitted to the user equipment using the weights. That is, the directivity for signal transmission is determined using a signal from the user equipment as feedback information.
In a W-CDMA method, which is being standardized by the 3GPP, HSDPA (High Speed Downlink Packet Access) is defined to realize high-speed data transmission. HSDPA is a communication method of offering a maximum transmission rate of 14 Mbps at a downlink and performs adaptive code modulation for adaptively changing QPSK/16-QAM, etc. depending on the radio environments between a radio base station and a user equipment.
In the following patent documents 1 through 4, technologies regarding the 3GPP, an antenna having directivity and HSDPA are described.
The patent document 1 describes a technology of controlling transmission power in a radio base station based on information about communication quality notified from a user equipment, in the 3GPP system. The patent document 2 describes a technology of implementing signal transmission having individual directivity for each user equipment, in a W-CDMA system. The patent document 3 describes a technology of individually controlling the directivity of the transmission beams of a data channel and a control channel, in a W-CDMA system. The patent document 4 describes a technology of controlling the transmission from a radio base station to a user equipment based on the communication quality at the time of both data receipt and data waiting, in a W-CDMA system.
[Patent document 1] Japanese laid-open patent publication No. 2002-325063 (FIG. 1, paragraphs 0042 and 0043)
[Patent document 2] Japanese laid-open patent publication No. 2001-339758 (paragraphs 0002 through 0007)
[Patent document 3] Japanese laid-open patent publication No. 2003-298498 (FIG. 2, paragraph 0021)
[Patent document 4] Japanese laid-open patent publication No. 2003-318861 (Abstract)
In HSDPA, uplink data from a user equipment to a radio base station is transmitted via a dedicated channel, while downlink data from a radio base station to a user equipment is transmitted via a shared channel. Specifically, on the Uu interface, the uplink data is transmitted via an HS-DPPCH (HS-dedicated physical control channel), while downlink data is transmitted via an HS-PDSCH (HS-physical downlink shared channel), as shown in FIG. 4.
A dedicated channel is a channel occupied by one user or one user equipment. Therefore, in a case where data is transmitted via the dedicated channel, a radio base station can enhance the communication efficiency as a whole system by directing the directivity of an adaptive array antenna toward the position of the user equipment.
A shared channel, on the other hand, is a channel occupied by a plurality of users or a plurality of user equipments. Therefore, in a case where data is transmitted via the shared channel, a radio base station must communicate all the user equipments that share the shared channel. Accordingly, in this case, a base station transmits the identical signals to the whole sector without causing a transmission beam to have directivity as shown in FIG. 5.
In a case where HSDPA is selected in the 3GPP system in this way, a shared channel is used for the transmission of downlink data from a radio base station to a user equipment so that the radio base station transmits a signal using uniform electric power in all the directions in a corresponding radio communication sector. Accordingly, power consumption in the radio base station becomes larger in comparison with the case of transmitting a signal having directivity in a specific direction. Especially, in HSDPA, a data transmission rate is high and transmission power is large so that the influence given to other channels as an interference wave is large. Furthermore, when signals are uniformly transmitted in all directions in a sector, the number of reflection waves increases as shown in FIG. 6 and the interference problem under a multi-path environment cannot be ignored. This problem is of importance in an urban area where there are many high-rise buildings.